EJB - MDB (and WSAD)

[no]

I am testing EJB MDB with WSAD (5.1.2), using the embedded JMS provider
and local test server.

I send 10 messages, and they are processed correctly in order by the
MDB when I set (within the administrative console) the listener port
parameter "Maximum Sessions" value to 1.

Now, I would like to process multiple messages simultaneously
(that's what MDB are made for, aren't they ?), so I tried to set
"Maximum Sessions" value to 10.
Then, the MDB processes 2 messages simultaneously (2 by 2 and not 10 by
10).

Is there another parameter somewhere to change ?
Is that a restriction imposed by the local test server ?

Thanks.